Field of the Invention
The present invention relates to a radiation imaging apparatus, a radiation imaging method, and a radiation imaging system, which are used for capturing radiation images.
Description of the Related Art
As radiation imaging apparatus, which are configured to capture a radiation image of an object with the use of radiation (e.g., X ray) transmitted through the object, radiation imaging apparatus capable of displaying a radiation image in real time have been widespread. Further, flat-panel radiation imaging apparatus (FPD) have also been proposed.
The FPD includes minute radiation detectors aligned on a quartz glass substrate in matrix. The radiation detectors each includes a lamination of a scintillator configured to convert radiation into visible light and a solid-state photodetector in which an amorphous semiconductor is sandwiched by a transparent conductive film and a conductive film. Further, solid-state photodetectors using a photodetector, e.g., a CCD or a CMOS, have been known. Further, radiation detectors configured to directly detect radiation with solid-state photodetectors without using any scintillator have been known.
The FPD is configured to detect a dose of radiation applied during a given accumulation time as a charge amount. Thus, when a radiation image of an object is captured, if charges irrelevant to application of radiation are present in the radiation detectors, the charges are superimposed on the radiation image as noise, with the result that the image quality of the radiation image is degraded.
For example, as charges causing noise, there are given residual charges that remain, after a radiation image is captured in previous imaging, based on the characteristics of the solid-state photodetectors and the scintillators. Further, as charges causing noise, there is dark current caused by charges generated mainly due to temperatures of the solid-state photodetectors. Besides, fixed noise caused due to defects unique to the radiation detectors is a factor to degrade the image quality of radiation images.
When a radiation image of an object is captured, as accumulation time of charges in which radiation is applied elapses, residual charges and charges of dark current components are also accumulated in proportion to the accumulation time, to thereby degrade the image quality of the radiation image. Thus, an offset correction process is performed in capturing a radiation image of an object in order to remove offset components due to residual charges and dark current charges accumulated during imaging, fixed noise, and other factors.
In general, the offset correction process is performed as follows: image data acquired without radiation application (non-exposure image data) is used as offset correction data, and the offset correction data is subtracted from a radiation image. In this case, capturing of radiation images of an object and acquisition of non-exposure image data (offset correction data) of the object are alternately performed, and the offset correction data is subtracted from the radiation images, to thereby perform the offset correction process. Further, non-exposure image data acquired before capturing of a radiation image of an object is used as offset correction data, and the offset correction data is subtracted from the radiation image, to thereby perform the offset correction process.
When capturing of radiation images of an object and acquisition of non-exposure image data (offset correction data) of the object are alternately performed, while afterimages can be reduced, there is a problem in that frame rates are decreased.
When offset correction data is acquired before capturing of a radiation image of an object, while frame rates are increased to enable high-speed continuous image capturing, e.g., moving image capturing, there is a problem in that afterimages cannot sufficiently be reduced. Further, dark current charges are changed due to temperatures of the radiation detectors, imaging conditions, or deterioration of sensors with time. Thus, there is a problem in that the accuracy of the offset correction process may not be sufficient when offset correction data is acquired before capturing of a radiation image of an object.
In this case, noise components are amplified depending on setting parameters such as output gains and frame rates. Further, afterimages are liable to be generated depending on imaged parts. Radiation images of imaged parts including metal or contrast agents have edges, and hence afterimages thereof are liable to be generated. Besides, in a technique using a plurality of radiation images, e.g., tomosynthesis or long-length imaging, afterimages are liable to be generated and diagnosis is hindered by the afterimages.
Further, in an ordinary FPD, dark current charges are liable to be unstable immediately after radiation detectors start driving and immediately after radiation application. Further, it has been known that residual charges generated after radiation application greatly change immediately after radiation application is finished. Thus, in order to stably perform the offset correction process, it is necessary to secure a certain period of time from start of driving of the radiation detectors to radiation image capturing or from previous radiation image capturing to next radiation image capturing. Meanwhile, it is desired that radiation images be captured in a short period of time immediately after the driving start or immediately after previous radiation image capturing in order to improve the operability of the radiation imaging apparatus.
In each of Japanese Patent Application Laid-Open No. 2012-183241 and Japanese Patent No. 4557697, there is proposed a technology of determining the stability and a variation amount of offset correction data with respect to time, to thereby acquire offset correction data, in order to maintain the accuracy of an offset correction process.
However, a determination unit configured to determine the stability and a variation amount of offset correction data, and a calculation unit configured to calculate those elements are needed, and hence a memory capacity and a circuit scale may be increased depending on image sizes, determination criteria, and frame rates.